1. Field of the Invention
The present invention relates to an image pickup apparatus such as a video camera, a control method therefor, and a program for implementing the control method.
2. Description of the Related Art
FIG. 7 is a block diagram schematically showing the system configuration of a conventional image pickup apparatus.
In FIG. 7, the conventional image pickup apparatus is comprised of a fixed first lens group 101, a zooming lens group 102 that performs zooming, an aperture 103, a fixed second lens group 104, a lens group 105 (hereinafter a “focus compensation lens”) having both a focus adjusting function and a so-called compensation function of correcting the movement of the focal plane due to zooming, an image pickup device (a CCD (Charge Coupled Device)), a zoom drive that drives the zooming lens group 102, a focusing drive 111 that drives the focus compensation lens group 105, and a CDS (Correlated Double Sampling)/AGC (Automatic Gain Control) circuit 107 that amplifies an output from the CCD 106.
A camera signal processing circuit 108 converts an output signal from the CDS/AGC circuit 107 into signals that are adapted to a recording device 109 and a monitor device 115 equipped with a display function. The recording device 109 records moving images and still images, with a storage medium such as a magnetic tape or a semiconductor memory.
An output signal from the CCD 106 is passed through an AF (Auto Focus) gate 112, which sets a range from which an optimum signal for focusing is to be extracted from within the entire screen. The size of the gate can be varied, and in some cases a plurality of gates are provided.
An AF signal processing circuit 113 extracts high-frequency components, low-frequency components and luminance difference components (that is, the difference between maximum and minimum values of the luminance level of a video signal) used for focus detection.
A camera/AF microprocessor 114 controls the zoom drive 110 and the focusing drive 111, based on an output signal from the AF signal processing circuit 113, and also controls a zoom switch 116 and an output signal from the camera signal processing circuit 108.
In the image pickup apparatus configured as shown in FIG. 7, the AF microprocessor 114 moves the focus compensating lens 105 so that the output signal level of the AF signal processing circuit 113 becomes the maximum, thus carrying out auto focus adjustment.
Next, a description will be given of an AF operation.
A TV signal AF method is mainly used in an auto focus device for video cameras. The TV signal AF method detects the sharpness of the screen from a video signal obtained by photoelectric conversion of the subject image by an image pickup device or the like and moves the focus lens so that the detected sharpness value, that is, an AF evaluation value, becomes the maximum, thus carrying out focus adjustment.
As the AF evaluation value, generally, high-frequency components of the video signal extracted by a band pass filter that allows only a certain frequency band to pass are used. The high-frequency components of a video signal obtained by shooting a subject image are typically such as shown in FIG. 8. The point at which the AF evaluation value becomes the maximum is the in-focus point, as shown in FIG. 8.
A description will be given of AF control carried out by the camera/AF microprocessor 114 during shooting of a moving image, with reference to a flow chart shown in FIG. 9.
In FIG. 9, when a moving image AF process is started, first, the focus compensation lens 105 is driven by a fine amount (microactuation) (step S101).
Next, it is determined whether or not the microactuation has brought the subject image into focus (step S102). If it is determined that the subject image has not been brought into focus, then it is determined whether or not the moving direction of the focus compensation lens 105 has been discriminated as a focusing direction (step S103). If it is determined that the moving direction of the focus lens 1505 has not been discriminated as a focusing direction, the process returns to the step S101. By contrast, if it is determined that the moving direction of the focus lens 1505 has been discriminated as a focusing direction, then the focus compensation lens 105 is driven at a high speed in a direction in which the AF evaluation value increases using the mountain climbing driving method (step S104).
Next, it is determined whether or not the AF evaluation value has passed a peak due to mountain climbing drive (step S105). If it is determined that the AF evaluation value has not passed a peak, the process returns to the step S104 and mountain climbing drive is continued. By contrast, if it is determined that the AF evaluation value has passed a peak, then the focus compensation lens 105 is driven in the opposite direction (step S106) to return the AF evaluation value during mountain climbing drive to the peak. Then, it is determined whether or not the AF evaluation value has reached the peak (step S107). If it is determined that the AF evaluation value has not reached the peak, the process returns to the step S106, where the operation of returning the AF evaluation value to the peak is continued. By contrast, if it is determined that the AF evaluation value has not reaches the peak, the process returns to the step S101, where the focus compensation lens 105 is driven by macroactuation to search for an in-focus position of the next moving image.
If it is determined in the step S102 that the subject image has been brought into focus, then the focus compensating lens 105 is stopped (step S108), the AF evaluation value when the subject image came into focus is stored (step S109), and a process for restart determination is carried out. That is, the last AF evaluation value stored in the step S109 and the present AF evaluation value obtained in the step S110 are compared, and if the difference between the two values exceeds a predetermined level, it is determined that restart is required (step S111). If as a result it is determined that restart is required, the process returns to the step S101 to start macroactuation drive operation. By contrast, if it is determined that restart is not required, then the focus compensation lens 105 is stopped and held in its current position (step S112), and thereafter, to accommodate a subsequent moving image, the process returns to the step S110 and the restart determination process is continued.
In the above described way, the camera AF microcomputer 114 drivingly controls the focus lens 105 by repeatedly carrying out the sequence of processes of restart determination→microactuation→mountain-climbing driving→microactuation→restart determination, such that the AF evaluation value is constantly maximized.
On the other hand, as a method widely used in single lens reflex cameras employing silver salt film, there is a phase difference detection method. According to the phase difference detection method, a light beam passing through the exit pupil of a taking lens are divided into two light beams, the two light beams are each received by a pair of focus detecting sensors, and an offset amount between output signals from the sensors, that is, a relative position offset amount between the two light beams in the direction of division of the light beam is determined according to the received amounts of light beams, to thereby determine the amount of offset of the taking lens in the focusing direction in a direct manner. Therefore, by carrying out a single accumulating operation using the focus detecting sensors, the amount and focusing direction offset are obtained, making a high-speed focus adjustment operation possible. It should be noted that, to divide the light beam into two light beams and obtain signals corresponding to the respective light beams, generally two systems each consisting of a focus detection optical system and a sensor are provided.
An AF method using an external ranging sensor includes, as a passive method, a method using a principle as shown in FIGS. 10 and 11. The external ranging sensor has first and second light paths, each having an image forming lens and a photo-detector array. Two signals of formed images of a subject passing through the first and second light paths are read out from the photo-detector arrays, and the correlation between the two subject image signals is calculated through an arithmetic operation. Specifically, it is arithmetically determined how many pixels one of the subject image signals should be shifted to make the correlation between both signals the maximum, to obtain distance information based on the amount of shift calculated by the arithmetic operation, using the principle of triangulation.
This arithmetic calculation principle is such that, first, as shown in FIG. 11, a shift amount X at which the correlation is maximized is calculated, and then, as shown in FIG. 10, a distance L to the subject is calculated from a distance B between a known focal distance f and the two optical path optical systems by proportionality calculation.
Moreover, as an active method, there is a method in which a propagation speed is measured using an ultrasound sensor, and a method in which triangulation is carried out using an infrared sensor widely used in compact cameras.
Furthermore, an auto focus device that is a combination of the external ranging method or the phase difference detection method and a TV signal AF method has been proposed (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. H05-064056 and Japanese Laid-Open Patent Publication (Kokai) No. 2002-258147). This auto focus device calculates the distance and direction to the in-focus position using the phase difference detection method, moves the lens to the in-focus position, and corrects any focus offset using the TV signal AF method. The method using a combination of the TV signal AF method and the external ranging method has the advantage that the focusing direction can be instantaneously determined if distance information is used in the mountain climbing operation according to the TV signal AF method, as well as the advantage that the focus lens can be moved in the focusing direction while confirming the distance to the subject during the mountain climbing operation. As a result, the distance information is effectively used to enable a high-speed focusing operation, and at the same time high-accuracy focusing can be achieved by an AF operation using an AF evaluation value obtained by the TV signal AF method.
However, the image pickup apparatus equipped with the auto focus device that is a combination of the TV signal AF method and the external ranging method has not been conventionally subjected to studies as to the electric power consumed by the external ranging sensor.
In the mountain climbing operation of the TV signal AF method, although the use of the distance information is very effective, in the vicinity of the in-focus position, a focusing operation can be achieved with sufficiently high accuracy using only the AF evaluation values of the TV signal AF method, and the distance information is not always required. Moreover, during a zooming operation as well, with zooming in the direction from wide angle to telephoto, the cam locus is dispersed, and with the AF evaluation value of the TV signal AF method AF alone it is difficult to continue to accurately grasp the distance to the subject due to the effects of change in the angle of view among other things. Therefore, the use of the distance information is effective. Conversely, with zooming from telephoto to wide angle, the cam locus is converged, and if the subject image is brought into focus at the start of the zoom, then by tracking the cam locus identified at that time point, it is easy to maintain the in-focus state even while zooming, and hence the distance information is not always required.
Thus, as described above, cases where the distance information is not essentially required include, for example, a case where the subject image is focused, a case where an in-focus estimating operation is carried out using only the AF evaluation value, and a case where zooming is carried out from telephoto to wide angle. Conversely, cases where the distance information is required include, for example, a case where it is immediately after the power is turned on, a case where it is immediately after switching from MF (Manual Focus) to AF, a case where searching for the direction in which the AF evaluation value increases to the maximum value, a case where fluctuation of the AF evaluation value is great, and a case where zooming is carried out from wide angle to telephoto.
If the external ranging unit is constantly supplied with electrical power, then the distance sensor (ranging sensor) wastefully consumes power even when the distance information is not always required as stated above.